1. Field of the Invention
The present invention relates to a method and an apparatus for creating an output of an imagesetter with high-resolution quality at the speed of low resolutions. The invention can be applied to imagesetters like direct-to-plate imagesetters, more specifically direct to flexo plate or direct to thermal offset plate imagesetters.
2. Description of the Prior Art
Imagesetters are apparatuses in which graphic sheets, such as films or plates, are exposed to light, e.g. by means of a laser spot, in order to create an image on these films or plates by moving one or more light beams over the surface of the film or plate.
Flexo platesetters are typically external drum engines because exposing a flexo plate in an internal drum is cumbersome (due to the thickness of the plate). They have typically a size of 40 by 60 inch, and flatbed technology on such big sizes is known to be limited in optical quality, certainly for such high laser power as is needed for imaging flexo plates. Moreover, a tendency is visible towards direct exposure of the flexo plate mounted on a sleeve, which will be processed in the round to use on the flexo press without dismounting the plate from the sleeve. As flexo printing plates are always mounted externally around the sleeve (to be able to print), it will be clear for the reader that the external drum technology is the most promising technology for flexo platesetters.
In the following, the circumference direction of the drum of the imagesetter is named the "fast scan direction", and the axial direction is named the "slow scan direction". The axial distance between two neighboring tracks is named the "advance". Under "addressability" is understood the number of positions a laser beam can occupy in one direction or the other. "Resolution" means the mean distance between two neighboring imaging positions.
As is known in the art, the light beam which creates an image on the plate in a single beam apparatus writes a first track in the fast scan direction, is then moved by the advance in the slow scan direction, writes a second track in the fast scan direction, etc . . . . The speed of the imagesetter, and thus the productivity, is directly proportional to the drum turning speed and to the advance.
Compared to a conventional film imagesetter, the productivity of a direct to flexo plate imagesetter is up to one order of magnitude lower when the same resolution is obtained because usually the needed laser power is much higher for a flexo plate imagesetter than for a film imagesetter. Therefore, in order to get a competitive product, the speed of a direct to flexo plate imagesetter has to be increased without loss of resolution.
The current invention is especially of high importance for plate imagesetters where laser power is the limiting factor. In practice, this is most often the case for computer to flexo applications (CDI.RTM. or engraving) and for thermal plate exposure. CDI.RTM. asks for a laser power of 2.5 J/cm.sup.2, so decent speeds can only be reached with lasers of 10 W and (far) more. In this region, every Watt more is a high additional cost. As a result, an imagesetter is needed which uses laser power in the most economic way: just enough to expose the plate, but nothing more.
The speed of an external drum imagesetter is given by following formula: ##EQU1## with nbeams the number of beams of the imagesetter, drum.sub.-- speed the rotation speed of the drum,
advance the axial distance between neighboring tracks, PA2 drum.sub.-- Perimeter the perimeter of the drum
Example: An imagesetter with a single beam, rotating 2000 rounds per minute with an advance of 1/2000 inch will expose 1 inch per minute around the drum. Suppose a drum of 40 inch perimeter, then 40 inch.sup.2 /minute are exposed or 4.3 cm.sup.2 /sec.
Increasing the speed is done through increasing one of the factors:
Increase the number of beams of the imagesetter. A multiple beam imagesetter is then obtained. Creating a multiple beam imagesetter is the typical way to increase the speed of an external drum imagesetter. However, it leads to a big number of additional difficulties such as consistent positioning of the different beams, avoiding interference and more complicated optics. PA1 Increase the drum speed. This is the typical increasing factor for internal drum machines. However, for external drum, increasing the speed is limited because the plate or film is to be held on the drum while rotating. Faster rotation asks for a higher force to avoid the plate to detach from the drum. At a certain speed, vacuum or other attachments are too weak to keep the plate safely on the drum. Typical limiting speeds are around 2000 rpm. PA1 Increase the drum perimeter. This is also limited to practical limits (the imagesetter is getting big) and plate size limitations. Also increasing the drum perimeter leads to higher forces needed to keep the plate on the drum for the same rotational speed. PA1 Increase the advance. This is the interesting point for the current invention. The classic way to increase the advance is to lower resolution. Lowering resolution has a far-going effect on the quality of the screen points. High quality flexo (which is the market of the direct flexo plate imagesetter) asks for a quality of 2000 ppi or equivalent. It is an object of this invention to produce a screening quality comparable to 2000 ppi with advances of 1/1000 inch or bigger. PA1 (a) receiving inputs organized as a number N of tracks with M sample points on each track, PA1 (b) converting the N.times.M image points into N2 non-straight tracks of M2 sample points each, whereby N2 is smaller than N and whereby M2 may be smaller than, equal to or larger than M, and PA1 (c) imaging the N2 non-straight tracks on the graphic sheet. PA1 (b1) different scan lines are taken together to form a track, whereby neighboring pixels of scan lines forming a track form a set of pixels in the slow scan direction, PA1 (b2) for each track, and therein for each set of pixels in the slow scan direction, the position of the light beam spot is selected depending on the values of said set of pixels, and PA1 (b3) subsequent spots are imaged based on the path formed by the subsequent positions of the light beam spot. PA1 means for mounting a graphic sheet, PA1 input means for accepting graphical image data organized in a rectangular grid of N rows and M columns, PA1 converting means for converting the graphical image data to N2 non-straight scanlines of M2 pixel positions, N2 being smaller than N, and M2 being smaller than, equal to or larger than M, PA1 at least one optical head(s) for emitting each a light beam suitable to write an image on a graphic sheet, preferably a laser head, PA1 primary beam deflection means for deflecting to the graphic sheet each light beam coming from the optical head(s), PA1 focusing means for focusing each light beam on the graphic sheet, PA1 secondary beam deflection means for deflecting each light beam in a suitable way for obtaining the productivity enhancement in the slow scan direction.
In the prior art, the problem of increasing the productivity of a direct to flexo plate imagesetter has been solved in different ways.
A first solution is to work in low resolution by using big pixels. Using the same low resolution in the fast scan direction as in the slow scan direction gives an overall low resolution. In that case the direct to flexo plate imagesetter is competitive with the conventional imagesetters for as far as speed is concerned, but it is not for as far as quality is concerned.
A second solution uses of pixels which are smaller in the fast scan direction than in the slow scan direction. The use of such rectangular pixels enables an increase of the productivity of the imagesetter, as the advance is larger, but it makes it difficult to form dot shapes. Furthermore a total new screen database is needed, and a lot of work has to be done to let the screens support it.
Another solution is described in U.S. Pat. No. 5,392,061 disclosing a system of "resolution enhancement". It is not a speed enhancement, but it is a quality enhancement, only in the fast scan direction. A format controller determines if an image pixel resides at an edge of an image feature and, if so, selects a pixel modification to enhance a representation of the feature edge.
Generally, by using a larger spot of laser light, the advance can be made bigger, so that less time is needed to image a whole plate, which leads to greater productivity. The advance can be made as big as the spot size is. But the larger the spot size, the less print quality can be obtained, because with a large spot size very small structures cannot reproducibly be imaged.
For a photopolymer printing plate for example, the minimum structure that can be reproducible imaged has, with the technologies used at present, a dimension of about 30 .mu.m. Therefore the final print quality cannot be further increased by shrinking down the diameter of the laser spot far below 30 .mu.m.
On the other hand, the outer shape of the imaged structures and therefore the final print quality will be influenced by the accuracy that is used to position the center of the laser beam focus spot during the imaging process (especially for slanted objects, screening dots, small text, . . . ). More than by spot size limitations, the final quality is influenced by addressability. A same spot size of 25 micron will make far better quality with 2000 ppi addressability than with 1000 ppi addressability.
Most of the current direct to flexo imagesetters use a spot size bigger than the advance when imaging in high resolutions such as 2000 ppi. This slows down the imaging process and does not make use of the speed advantage.